This invention relates generally to electronic circuit board design for systems with multiple circuit boards which are connected to and maintained in common alignment by a backplane circuit board. More particularly, this invention is directed to an arrangement that allows the circuit boards to communicate digital data between one another without physical connection between the circuit boards, and without any modification of or design impact on the backplane circuit board used to host the circuit boards.
Today many electronic systems, including computer systems, physically consist of several circuit boards, or cards, that plug into a common xe2x80x9cbackplanexe2x80x9d circuit board. The circuit boards are usually multi-layer printed circuit (xe2x80x9cPCxe2x80x9d) boards with discrete and integrated components that are soldered or otherwise physically attached to and in contact with xe2x80x9ctracesxe2x80x9d which are conducting paths connecting the devices to form the electronic circuit. The backplane is usually another PC board with sockets to hold the multiple circuit boards, and which has conducting paths connecting the pins in the sockets, and thus the circuitry on the circuit boards with one another. The assembly of circuit boards that plug into a backplane is often held in an equipment shelf that uses slots or rails to guide the circuit boards and keep each aligned with its associated socket on the backplane during insertion or removal.
One of the problems with this configuration is that the backplane traces and thus the connections between the circuit boards are fixed in hardware. Once the backplane has been designed, there are limits on the types of changes that can be made to the signal paths between the circuit boards. If the system designer desires additional bandwidth or types of connectivity between circuit boards, and these additional accommodations are not supported by the backplane configuration, the designer must resort to additional hardware or to redesigning the backplane to satisfy the new need. Such remedies may include a fiber-optic link between the circuit boards, paddleboards that connect to wire-wrap pins on the reverse side of the backplane, or ribbon cables that connect to sockets on the edges of the boards.
Redesigning the backplane is undesirable because it increases the number of proprietary-specific backplane configurations thus reducing circuit board interchangeability. Specialized backplanes require specialized boards to fit those backplanes, and this means that neither the backplane nor the boards themselves can be purchased xe2x80x9coff the shelfxe2x80x9d from third party vendors. Redesigining hardware also slows the product development cycle and increases the time between product concept and market availability.
Adding hardware links between circuit boards in a shelf makes assembly and maintenance of the finished product much more complex. In order to replace a board in a shelf, all connections must be removed before the circuit board itself can be removed. The connectors between the circuit boards add additional points of failure, and reduce the reliability of the boards and thus the system. Replacing a faulty circuit board in the shelf takes longer when there are cables or fibers connected to the board that must be manipulated, increasing the time required to repair a fault. And sometimes the links between circuit boards may be xe2x80x9cdaisy chainedxe2x80x9d so that removing the link from any board may isolate other unrelated boards from one another.
If a specific hardware link is created between circuit boards, either through conductors in the backplane or using cables or fibers connecting the boards, the characteristics of that link become quite fixed in nature. It is generally difficult to change the type of data, bandwidth, or function of a hardware link simply because the hardware is designed to provide a path for a very specific type of data, bandwidth, or function. The hardware connection thus typically reduces the flexibility of the design of the circuit board and the system. As an example, if a two-conductor Ethernet connection is provided between two circuit boards in an existing configuration, it is very difficult to redesign the hardware connection to instead provide a high-speed mate-update bus between the two circuit boards. An additional hardware path would be needed to provide this function. Thus, the physical parameters and characteristics of a circuit board connection affects the functions available in that connection.
What is needed is a generic, flexible connection technology between circuit boards in a backplane that provides a data path between the boards, which is independent of the backplane, and which requires no physical connection between the boards. Accordingly, it is the object of the present invention to provide a data path between circuit boards in a backplane with these characteristics.
The invention uses free space optics to provide a communication path between circuit boards installed in a backplane in an equipment shelf. One or more circuit boards in the shelf have one or more optical transmitters that emit a beam of light which may be in the form of a laser beam. One or more circuit boards in the shelf also have an optical receiver for receiving light emitted from the transmitter on one or more of the other boards. Circuit boards may also be provided with optical splitters and optical combiners, and may have apertures drilled through the boards for allowing light signals to pass through. Circuit boards may also have small lenses and/or opaque pieces of material to change the physical characteristics of the optical path. The optical signal transmitted through unobstructed free space between circuit boards is used as a high-speed carrier of (typically digital) information between the boards. The present invention can be compared to a fiber optic carrier system, except that it does not use fibers to carry the signals between the circuit boards. Rather, the present invention transmits signals between circuit boards through free space, depending on the alignment of transmitters and receivers to make the connection. Since circuit boards are typically xe2x80x9clockedxe2x80x9d into place in the equipment shelf using latches, the boards are designed so that when they are installed, the receivers and transmitters on the boards are aligned and a signal path is established. When a circuit board is removed, all connections to and from that board are automatically broken.
In one embodiment of the present invention, a pair of circuit boards are arranged in spaced, parallel alignment connected to a backplane circuit board. A first circuit board includes an optical transmitter, while the second circuit board includes an optical receiver. The optical transmitter directs a light beam onto the optical receiver to permit communication between electronic circuitry on the two circuit boards. The optical transmitter is preferably a laser. The second circuit board may also be provided with an optical transmitter, while the first circuit board is also provided with an optical receiver. The optical transmitter on the second circuit board directs a second light beam onto the optical receiver on the first circuit board to allow for bi-directional optical communication between the two circuit boards. A second, opposed side of either the first or second circuit boards may similarly be provided with an optical transmitter and receiver pair for communicating with a similarly configured third circuit board. This arrangement allows a pair of adjacent circuit boards to communicate together, or the circuit boards may be used in a daisy-chain approach that allows any circuit board to communicate with any other circuit board via one or more intermediate circuit boards. In this latter embodiment, the circuit boards may be designed so as to be capable of determining in which slot in an equipment shelf the circuit board has been inserted to permit the daisy-chain approach.
Another embodiment of the invention contemplates a circuit board inserted in an end slot in an equipment shelf which serves as a xe2x80x9chubxe2x80x9d. This end-most circuit board includes an optical transmitter. Other circuit boards are inserted in the equipment shelf and are arranged in a spaced, aligned array. These latter circuit boards are each provided with an aperture which permit a light beam emitted by the optical transmitter on the end-most circuit board to pass through the other circuit boards. Each of the of the latter circuit boards further includes an optical splitter disposed adjacent the aperture in the board and aligned so as to receive a light beam. This allows the hub circuit board to provide information to each of the remaining circuit boards. This embodiment further contemplates the hub circuit board being provided with only an optical receiver. An optical transmitter on each of the other parallel, spaced circuit boards directs light beams through apertures in each of these latter circuit boards onto the optical receiver on the hub circuit board. Disposed adjacent each aperture in the latter circuit boards is an optical combiner which allows the signals from all of the circuit boards xe2x80x9cupstreamxe2x80x9d to be combined to form a composite signal which is directed onto the optical receiver of the hub circuit boards. The optical signal of each circuit board distinguishes it from the remaining circuit boards, such as on the basis of frequency or data format, to permit the optical receiver to determine which circuit board transmitted a received signal. This configuration can be combined with the previously described configuration to provide an optical data bus whereby each circuit board can receive information transmitted by a bus circuit board, and each board can also transmit information back to the bus circuit board.